1. Field of the Invention
This invention relates to a method of producing a thin plate of a sintered rare earth magnetic alloy having a hard ferromagnetic phase surrounded by a readily grindable grain boundary phase. The thin plate is called as a wafer in the specification.
2. Background Art
Sintered rare earth magnetic alloys composed mainly of Nd—Fe—B are considered to have a metallic structure consisting a ferromagnetic phase whose main phase is Fe14Nd2B and, surrounding the ferromagnetic phase, a Nd-rich grain boundary phase (nonmagnetic or soft magnetic phase). These alloys can be used to produce high-performance magnets having an energy product (BHmax) of not less than 35 (MGOe). Various improvements have been achieved with respect to the poor corrosion resistance and oxidation resistance that have long been a matter of concern regarding these magnets, and also with respect to their various properties such as the temperature-dependence of their magnetic characteristics and relative low curie point. Advances achieved up to now are impressive even as viewed solely from the structural viewpoint. These include, for example, sintered rare earth magnetic alloys that have part of the Nd replaced with another light rare earth element or a heavy rare earth element, others that use Co as an alloying element, and still others that contain C (carbon) or that are appropriately balanced with other alloying elements.
In addition, the emergence of numerous improved methods for producing sintered rare earth magnetic alloys is adding to the store of technologies enabling economical production of good quality sintered rare earth magnetic alloys. One resent result is the extensive use of sintered rare earth magnetic alloys in equipment at the heart of precision electrical products and the like.
The present invention is aimed at enabling production of excellent quality wafers made of such sintered rare earth magnetic alloys. As used in this specification, the term “sintered rare earth magnetic alloys” encompasses not only sintered rare earth magnetic alloys composed primarily of Nd—Fe—B but all types of rare earth magnet sintered bodies including, for example, ones that are structurally characterized in that they have part of the Nd replaced with another rare earth element, incorporate Co as an alloying element, include C (carbon), or contain other alloying element(s). In this specification, these are referred to collectively as “Nd-system sintered rare earth magnetic alloys” or in abbreviated form as “sintered rare earth magnetic alloy.” Typical of these are (Nd, R)—(Fe, Co)—(B, C)-system sintered magnetic alloys. Here, R designates rare earth elements other than Nd. All of these sintered rare earth magnetic alloys include magnetic crystal grains composed of an intermetallic compound. The magnetic crystal grains are surrounded by a (Nd, R)-rich grain boundary phase and a grain boundary phase containing a B-rich, Co-rich or C-rich phase. These grain boundary phases are generally softer and more brittle than the magnetic crystal grains composed of intermetallic compound. Although strictly speaking the composition of the intermetallic compound forming the magnetic crystal grains differs with the contained alloying elements, it is generally considered to be substantially Fe(Co)14Nd(R)2(B, C).
A sintered rare earth magnet of this type is typically produced by following production steps such as shown in FIG. 1. Although the magnet is sometimes given its final shape in the step of press molding the alloy powder before sintering, in view of productivity considerations it is usually formed as a rod or cylinder that is cut into the individual forms of wafer after sintering.
As an example, consider the case of producing a wafer such as a thin disk-shaped sintered rare earth magnet measuring several mm or so in thickness and 10 mm in diameter. First, fine powder obtained by pulverizing the alloy to a particle diameter of 10 μm or finer is press-molded into a round rod of a length of, for example, 30 mm. To allow for contraction during sintering, the diameter of the press-molded rod is made larger than 10 mm at this time. The molding is conducted in a magnetic field so as to align the powdered alloy particles. The alignment is sometimes in the axial direction of the rod, sometimes perpendicular to the axial direction, and sometimes radial. This alignment is carried out if an anisotropic magnetic is desired. Actually, it is almost always conducted, because sintered rare earth magnets usually exhibit high performance as anisotropic magnets. When an isotropic magnet is to be obtained, alignment is not conducted and the crystal orientation is therefore random. The rod-shaped sintered product may or may not be heat treated before being sliced into disks (wafers) of about 2 mm thickness. The disks are bored at the center (if necessary) and are then magnetized to obtain magnets of the desired shape.
The cutting of the rod into thin pieces is done by slicing. Conventionally the slicing of a sintered rare earth magnetic alloy is done using either an external blade formed by adhering abrasive grains to the outer peripheral surface of a metal disk or an internal blade formed by adhering abrasive grains to the inner peripheral edge of a metal disk center hole. The external blade is more commonly used. Since the hardness of a sintered rare earth magnetic alloy is extremely high, on the order of a Vickers hardness of 500 or greater, ordinarily Hv 600–1000, the slicing of sintered rare earth magnetic alloys has come to be widely done using the highly technically advanced external blade (saw blade) developed for silicone wafer slicing and the like.
In this connection, the assignee filed Japanese Patent Application No. 2000-117764 for an alternative cutting method to that using an external blade. In this cutting method, a flexible wire of not greater than 1.2 mm diameter is pressed onto the sintered rare earth magnetic alloy and the wire is moved axially while supplying to between the alloy and the wire an abrasive fluid composed of abrasive grains dispersed in a dispersion medium. This cutting method was found to be capable of cutting sintered rare earth magnetic alloy into thin slices at high yield.
Sintered rare earth magnetic alloys are capable of exhibiting outstanding magnetic characteristics as small magnets. The shapes and sizes of such magnets for use in precision equipment have therefore become increasingly compact. The accuracy of the precision machining required has risen in proportion. In the case of sintered rare earth magnetic alloys for use in the miniature motors and speakers installed in mobile phones and audio devices, for example, the thin magnet wafers (including disk-, doughnut-square-shaped and the like) have to be finished to a thickness of under 1 mm, often to around 0.5 mm, and a ratio of thickness to planar surface area ratio of 0.05 or less.
In such case, when the sintered rare earth magnetic alloy is sliced into thin wafers with a cutter, surface irregularities are likely to occur owing to the distinctive structure of the sintered rare earth magnetic alloy. Specifically, as pointed out above, the sintered rare earth magnetic alloy has an extremely high hardness of around Hv 500–1000 and, in addition, has a structure consisting of hard magnetic crystal grains composed of intermetallic compound dispersed in a soft grain boundary phase. Surface irregularities therefore occur because the magnetic crystal grains are not sliced through but remain sticking out from the surface from place to place (as though only the fine grains of the grain boundary phase were scraped off). Nicks, saw marks and the like are also apt to be formed in the cut surface. Owing to these circumstances, difficulty has been experience in slicing wafers exhibiting a flat, smooth surface from a sintered rare earth magnetic alloy.
The sintered rare earth magnetic alloy may be cut to a very thin wafer thickness of under 3 mm, or even under 1 mm. If the planar surface smoothness of the wafer is poor and the magnetized wafer magnet obtained from it is mounted on a component having a flat surface, gaps will remain between the magnet and the component surface. Strain will arise in the wafer owing to the strong magnetic force acting between the two (A sintered rare earth magnetic can achieve a BHmax of 35 MGOe or greater). The wafer may not have sufficient strength to resist the strain, in which case it will break.
Even if it does not break, its performance will be degraded by the lack of a flat surface owing to the adverse effect on the distribution of the magnetic flux density from the wafer surface. When a wafer magnet with inferior planar surface flatness is used in a small motor or speaker, for example, the unevenness of its magnetic force will produce irregular vibration. When it is used in a step motor, the gap between itself and the yoke will increase to cause magnetizing loss. In addition, defective bonding may occur when the magnet is mounted.